Phantom Mansion is a 2D puzzle game with enough depth and length of play that, with slightly better graphics and code that wasn't leaky (expect your machine to slow as you play) it could be a game you'd be willing to pay money for, if you're a fan of puzzlers. But you don't have to, since it's free.
Phantom Mansion is being developed as a series of 8 related Flash games, each representing a differently colored "door" in the eponymous Phantom mansion; four are currently available, with, presumably, the next four to come.
Each game provides an area of the Phantom Mansion where there are a dozen or more doors. Each door is, in essence, a level. In each level, you must collect all "souls" (represented by skull icons), and may not leave the level until you do. Typically, some areas of a level can only be unlocked with a key, and you must figure out how to get to the relevant keys. Your character is controlled by using the arrow keys; some squares in the level can only be traversed once (or twice, or three times) before they become death to enter (the conceit is that the floor boards have given way and you fall to your death). Other squares are death to enter unless you have a "magic lantern" that reveals "hidden bridges," with the magic lantern hidden somewhere in the level. Sometimes there are boxes that you can push into uncrossable squares to make them crossable; often, there are zombies who walk back and forth in predictable paths whom you must evade.
As with most puzzle games, in other words, some complicated and increasingly difficult puzzles are created out of a handful of simple elements. Over time, additional elements are introduced, but there's always a "scroll" somewhere on the level you can read to learn what they do. There's no "undo" or automatic level restart, but you can always kill yourself--this just takes you back to the current start of the level (not the start of the game, thankfully).
The puzzles are variable and clever enough to hold your interest; the theme is largely irrelevant to actual gameplay, but I suppose games such as this need some kind of theme. There is one notable flaw; it appears that the code is leaky, as you'll find that your machine (and the game) responds more slowly over time, perhaps making the game unplayable eventually. There's no explicit "save game," but your current status is scored on your machine as a cookie, so the work-around is simply to close your browser, restart, and go back to the last level you were trying to solve.